Silviculture

Overview

Silviculture refers to practices that are used to manage the growth and composition of forest vegetation for an array of objectives, such as wildlife habitat, timber, water resources, recreation, and much more. Many silvicultural practices are potentially valuable tools for helping forests respond to a changing climate.

This set of papers discusses some of the expected impacts on forest vegetation as a result of climate change, and how silviculture may be used to help forests adapt. Some detailed regional examples are discussed, and additional resources are provided to allow additional exploration.

Preparers

Douglas-fir forests of the western hemlock zone in the Pacific Northwest are some of the most productive in the world. Douglas-fir (Pseudotsuga menziesii) typically dominates as a result of disturbance over the past 150 years, however, the shade-tolerant western hemlock (Tsuga heterophylla) is a potential climax species and several conifer associates occur in varying abundance (19). Hardwood associates including red alder (Alnus rubra), bigleaf maple (Acer macrophyllum) and others occur in relatively low abundance.

The western hemlock zone is extensive, encompassing the west slopes of the Cascade Range and all but the most coastal portions of the Coast Range (19). It extends from southern Oregon northward through Washington into British Columbia. The zone is generally characterized by a wet, mild maritime climate but it encompasses a great deal of climatic variation arising from latitude, elevation and location in relation to mountain ranges. Mean annual precipitation varies from 150 to 300 cm, with only 6 to 9% of precipitation occurring in summer. The amount of precipitation occurring as snow varies with elevation and ranges from a trace to an excess of 200 cm. Average annual temperatures range from 8 to 9 °C, with neither winter minimums nor summer maximums considered extreme.

Potential Vulnerabilities

Increasing temperatures combined with unchanging or decreasing summer available moisture will lead to more frequent water deficits when evaporative demand exceeds seasonal moisture availability. With increasing temperatures, productivity in water-limited systems is expected to decrease as water deficits constrain photosynthesis (1). Elevated CO2 concentrations may have a fertilization effect that may increase carbon gain per unit of water transpired. However, limited water supply could negate CO2 fertilization benefits (2). Much of the Douglas-fir forest at mid-to upper elevations is currently considered energy-limited rather than moisture-limited; productivity is constrained predominantly by light and temperature (1). Climate induced changes in energy-limited ecosystems may include warming effects of decreased snowpack depth and duration, coupled with increasing temperature. The fundamental limiting factors driving ecosystem composition, processes and productivity may be altered - particularly at ecotones between energy and moisture limited environments.

Douglas-fir is wide-ranging but displays substantial local adaptation, making the species responsive to climatic changes (3, 4). Current populations within the western hemlock zone differ considerably for adaptive traits, particularly bud phenology and emergence. Variation in bud-set, emergence and growth is strongly related to elevation and cool-season temperatures. Variation in bud-burst and growth partitioning to stem diameter versus height is related to latitude and summer drought. With increasing temperatures, suboptimal timing of growth may lead to water deficit susceptibility, failure to adequately develop autumn cold-hardiness leading to frost damage, and failure to achieve requisite chilling and forcing needed to break winter dormancy (5). Based on statistical models of climate suitability, by the 2060s, climate changes west of the Cascade crest will be sufficient to put Douglas-fir at risk of maladaptation, particularly at low elevations throughout its current distribution (1).

Climate-related vulnerabilities will arise not only from effects on tree vigor, but also from alterations of disturbance regimes associated with insects, diseases and fire. Across western North America there have been examples of large-scale insect epidemics causing vast areas of forest mortality. Although suitable climate for bark beetles exists throughout much of the Douglas-fir region west of the Cascade crest, typically bark beetle outbreaks have been less extreme than in the drier forest types east of the Cascades. Nonetheless, outbreaks of Douglas-fir beetles (Dendroctonus pseudotsugae) have occurred in the western Cascades of Oregon as recently as the early 1990's (6). Historical analysis revealed that landscape-scale factors such as prolonged drought and windstorm events were more important predictors of local outbreaks than were fine-scale factors such as individual tree growth rate, a proxy for host-tree vigor (6). Douglas-fir beetle epidemics occurred more in areas with a greater abundance of mature and old-growth conifer vegetation. Beetle densities were sufficient to overcome the resistance associated with individual tree and stand vigor.

Foliar diseases are among the pathogen groups most likely to be affected by changing climate. Swiss needle cast (Phaeocryptopus gaeumannii) was historically a minor leaf pathogen that has since the 1990s been a substantial pathogen of Douglas-fir plantations in the coastal fog belt of Oregon (7). Warmer winter and spring temperatures, if combined with potential increases in winter and spring moisture, could sustain high levels of Swiss needle cast infection (7). While typically non-lethal, Swiss needle cast can have major impacts on Douglas-fir growth and production in heavily infected stands (8).

Various root rot pathogens (e.g. Phellinus weirii) are tied to the distribution of Douglas-fir and other conifers. Because soil temperatures may be somewhat buffered from changes in air temperature, it is difficult to assess how a changing climate, coupled with altered species distribution, will affect the occurrence of root pathogens. However, one concern is that warmer winter temperatures could eliminate a climatic barrier to the widespread infection of Douglas-fir by Phytophthera cinnamomi, a root and stem pathogen that has severe impact on Douglas-fir in New Zealand (7).

Fire regimes of the relatively moist Douglas-fir forests of the western hemlock zone are historically characterized by low frequency, moderate to high severity events. While a regional average fire return interval of 230 years has been estimated, studies suggest variable return intervals over this geographically large and ecologically diverse area (9, 10). The great extent of Douglas-fir dominated forests arising between the 1850's and 1930's was due in large part to stand replacing fire events (10). With climate change, area burned in west-side Douglas-fir regions is projected to increase about threefold by the 2040's, but will still remain a relatively small portion of landscape (1).

It is important to recognize that potential vulnerabilities change with the stage of forest development. Regeneration is often considered the developmental stage most vulnerable to climate related stresses such as drought or high evaporative demand (2). Individual seedlings can lack the root system development and water and nutrient storage capacities to buffer against seasonal resource limitations. Natural regeneration may be hindered at a more fundamental level as alterations of weather patterns may result in a disruption of the coordinated processes of pollen dispersal and flower receptivity leading to poor seed set. Established stands may experience vulnerabilities that arise at the community level, such as competition for resources that is intensified by climate dynamics and associated stresses, such as insect or disease infestations. Older-aged stands may be buffered to some extent from climate variability but the vulnerabilities may be more subtle. Tree species may be differentially impacted by climate changes - potentially leading to a simplification of composition in mixed-species older-aged forests. It can be postulated that some elements of the diverse biological communities of arthropods and bryophytes that comprise distinct old-growth canopy communities may be threatened without detection.

The western hemlock and Douglas-fir zones of Washington and Oregon. Adapted from a map by Jan Henderson, USDA Forest Service.

This Douglas-fir tree plantation is in a stage of growth where a dense canopy results in low levels of light and minimal growth of understory vegetation. Credit: Paul Anderson

A Douglas-fir plantation five years following thinning from 325 trees per acre to 185 trees per acre. Thinning is one possible strategy for increasing forest resilience to climate change. Credit: Paul Anderson

Planted western hemlock and Douglas-fir seedlings develop into a second cohort of trees in a heavily thinned plantation on the Siuslaw National Forest. Planting can increase forest stand diversity. Credit: Paul Anderson

Thinning of a Douglas-fir plantation in western Oregon provides opportunity for diverse species to get established in the understory, through natural tree regeneration and planting. Credit: Paul Anderson

Management Options - Silvicultural Strategies

Density management

Density management to avoid excessive inter-tree competition may be one of the most useful means for increasing tree and stand vigor, and therefore forest resistance and resilience to climate related stresses. Relationships between stand density, growth, and site quality have been well-characterized for Douglas-fir forests of the Pacific Northwest. Degree of crowding and potential demand on site resources has been characterized by stand density indices such as Curtis Relative Density (11) or Reineke's SDI (12). Density management guidelines based on stand density indices have been useful for regulating stand productivity and for identifying thresholds for bark-beetle epidemics. Assuming that climate stresses are manifest as lower site potentials and lower thresholds for competition-related mortality, a conservative approach may be to maintain stands within a lower range of stand densities.

Management of overstory density alone may not be sufficient to enhance vigor and resilience. With overstory thinning, increased light and moisture resources can provide an opportunity for regeneration and growth of understory vegetation that places demands on site resources, offsetting to some extent the benefits derived from thinning. If untreated, biomass residues from thinning may provide breeding opportunities for insects (13), and fuel loads that increase flammability and fire risk (14). Silvicultural strategies need to account for the unintended consequences of any harvest operation.

Silviculturists have relied on site index to communicate differences in the potential productivity of a location. However, changes in climate may lead to altered site productive capacity. In the absence of sophisticated climate-site models, correlations between existing plant associations and site indices can provide a simple means to visualizing future changes in site productive capacity. A rudimentary sense of possible growth or production response to altered site capacity can be deduced from the site indices with plant associations occupying slightly drier or warmer environments.

Increasing Diversity

One way to manage uncertainty is to increase within-stand and landscape diversity by regenerating with mixtures of species. In stands or landscapes simplified by forest management practices or altered disturbance regime, encouraging the regeneration of a variety of species consistent with the potential natural vegetation may enhance the adaptive capacity. Alternatively at lower elevations where summer water deficits are likely to become more frequent, planting a proportion of drought tolerant species such as ponderosa pine (Pinus ponderosa), incense cedar (Calocedrus decurrens), or Oregon white oak (Quercus garryana) may provide a greater breadth of adaptive characteristics than a monoculture of Douglas-fir. Mixed species regeneration opportunities are not limited to clearcuts or burns, but may occur with partial overstory removals or forest gaps created in variable density thinning. Recent studies are beginning to provide useful information on the survival, growth and productivity of various conifer and hardwood species planted under the residual overstory of thinned Douglas-fir and western hemlock forests (e.g. 15). Root rot pockets and small windthrow gaps can provide local, small-scale opportunities to manage for alternative species, including more light demanding species or root rot resistant hardwoods, often at relatively low risk to near-term stand productivity. Furthermore, novel approaches to seedling deployment, such as small, widely dispersed clusters of various species can be embedded within conventional plantations or areas of natural regeneration.

Another approach to enhancing diversity and adaptability is planting seedlings comprised of mixtures of local and lower elevation or more southerly seed sources. For Douglas-fir, the analysis of St. Clair and Howe (4) suggests that to match current populations to the prospective climate of 2100, one might plant seedlings from sources that are 450 to 600 meters lower in elevation, or 1.8 to 2.5 degrees southerly in latitude, or a combination thereof. Similar relationships for species other than Douglas-fir are only beginning to emerge. However, for the 20 or so Northwest species with defined seed zones, the general guidelines for seed transfer are the same - from lower to higher elevation or from southern to northern latitudes. It is also important to recognize species differences in their degree of local adaptation. Whereas Douglas-fir demonstrates a relatively high degree of localized population adaptation, other widely distributed species such as western white pine are broadly adapted and show little localized adaptive population structuring. Initially, seed transfers for species with strong local adaptation might be restricted to adjacent seed zones, or perhaps more conservatively, to shifts between distant locations within a seed zone. Seed transfer guidelines could be more relaxed for those species demonstrating weak adaptive population structuring.

As with planting species mixtures, introductions of new seed sources should be practiced judiciously, either as a small proportion of any regeneration effort, or focused on those areas where the current population is most likely to become stressed. There is always some risk that introduced seed sources may be less fit than local sources in the near-term, and therefore they will only provide a future benefit if they survive. Where the expected lifespan of a stand is relatively short, such as a commercial forest plantation of Douglas-fir with a rotation age of 35 to 50 years, the risks associated with seed transfer may be offset by a relatively rapid population turnover. In contrast, the risks may be greater for forests managed with an extended rotation or late-successional reserve strategy in which population turnover opportunities arise infrequently or through unintended disturbance.

Maintain riparian ecosystems

Maintaining the integrity of aquatic and riparian ecosystems is a critical consideration for silvicultural adaptation strategies in the Pacific Northwest. Streamside vegetation is important to sustaining the distinct microclimate and habitat conditions associated with streams and riparian areas. The functionality of species composition and structure and width of streamside vegetation buffers is an active area of research. It is increasingly apparent that buffers are important to mitigating the near stream impacts of forest harvesting. The buffer configurations required will differ with stream orientation, flow regime, presence or absence of fishes, amphibians and other aquatic dependent organisms, local topography, vegetation composition and structure, and the intensity of harvest or other forest disturbance (16, 17). With projected warming and decreases in summer stream flow, fishes and other aquatic and riparian dependent organisms will potentially face contracting stream networks, increased stream temperatures, altered food supplies and other habitat conditions that challenge population fitness and ecosystem resilience (18).